In order to reduce the overall size, a box-like wiring substrate having a cavity in which an electronic component, such as a crystal oscillator, is mounted is required to reduce the thickness of side walls surrounding the cavity. For example, there has been proposed a package which has a plurality of via conductors formed in side walls, each via conductor having an arc shape as viewed in plane and partially exposed in the form of a flat vertical surface at an inner side surface of the cavity, and in which the upper ends of the via conductors are connected to a plurality of wire bonding pads, respectively, formed on a front surface surrounding the opening of the cavity and having the form of a rectangular frame as viewed in plane and in which the lower ends of the via conductors are connected to a plurality of electrode pads, respectively, formed on the bottom surface of the cavity (refer to, for example, Patent Document 1).
However, when the side walls which encompass the front surface surrounding the cavity and having the form of a rectangular frame as viewed in plane are reduced in thickness, the following problem has arisen in some cases: in melting brazing materials disposed on the electrically conductive layers of the plurality of wiring substrates in the multi-piece-wiring substrate for brazing metal frames onto the front surfaces, respectively, the adjacent brazing materials are connected in a bridging manner.
Meanwhile, in the case where, in order to establish electrical continuity between an electrically conductive layer formed on the front surface in the form of a rectangular frame and an electrically conductive layer formed on the bottom surface of the cavity, wall-surface electrically conductive layers are formed by printing on the side walls of the cavity for connecting the two electrically conductive layers, the following problem has been potentially involved: breakage of electrical continuity caused by the occurrence of an unconnected portion, a deterioration in productivity, or difficulty in brazing the metal frame horizontally onto the electrically conductive layer formed on the frame-like front surface due to protrusion of the wall-surface electrically conductive layer to a side toward the electrically conductive layer formed on the frame-like front surface.
Furthermore, in the case where the electrically conductive layer is formed across the overall width of the frame-like front surface, a portion of an Ni or Au plating layer covering the surface of the electrically conductive layer runs off the front surface and down to the outer surface of the side wall or to the inner side surface of the cavity; i.e., a so-called plating sag arises; therefore, in joining a metal lid onto the electrically conductive layer by resistance welding, electrical resistance is apt to change according to the amount of the sag. Accordingly, in some cases, an electronic component mounted in the cavity has failed to operate properly due to a defective seal between the plating layer covering the electrically conductive layer and a plating layer covering the surface of the metal lid caused by defective welding.